1. Field of the Invention
The present invention relates to a reception amplifier in a nuclear magnetic resonance system having an input transistor.
2. Description of the Prior Art
In nuclear magnetic resonance systems, a relatively high-power transmission pulse is forwarded by a transmission-reception diplexer to a transmission antenna. The transmission antenna also receives reception signals that are smaller than the high-power transmission pulse by orders of magnitude, and the reception signals are transmitted to a reception amplifier by the transmission-reception diplexer. However, a complete electrical separation of the reception amplifier during the transmission event is not possible because of the unavoidable switching capacitances in the transmission-reception diplexer. As a result, residues of the transmission signal proceed, by means of the transmission-reception diplexer, to the reception receiver. Previously, the reception amplifier had to be provided with field effect input transistors that have a high input impedance. As field effect transistors (FET's) are not sensitive to the currents arising from the transmission signal, only the reception signals are received by the reception amplifier. However, amplifier stages having FET's can only process relatively narrow-band signals.
Further, when a single broadband pre-amplifier is used to replace the various, narrowband reception amplifiers, a bipolar transistor must be used as an input transistor. However, due to the low input impedance of bipolar transistors, bipolar transistors can be gradually destroyed due to repeated exposure of the bipolar transistors to the current peaks arising from the transmitter.